I'm reminded of the fact this was the original plot for Reds! but the writers went back on it because they figured out the existence of the kind of movement capable of bringing communism to America would derail the isolationist trend in the USSR that made Stalin so dominant.
But if we go with a slightly later American revolution, it makes the USSR's degeneration more irreversible and Trotskyism more antagonistic so that probably works better.
I really wouldn't put the European empires under the US' wing though.The US always had at the very least a hypocritical pretense of an anti imperialist stance (even if it was a budding empire of its own). I think you can build on that. Meanwhile, Stalin had a tendency to seek accommodations with the liberal order as part of socialism in one country, especially with his reliance on the wallies to survive WW2.
I think you can engineer a world were trotskyism absorbs elements of OTL Castro and Che Guevara and blames the stalinist USSR for being too kind on the empires, while the USSR is mostly concerned with making inroads into European diplomacy through its satellite communist parties, who had some pretty decent performances OTL and were only irrelevant due to being cordoned off.
Cue trotskyists declaring popular fronts the instrument of the revisionist imperialists while European communist parties support Empire on grounds of "development".
If you want it to end up with a two blocs cold war, you can throw in Maoist China siding with the US, and you have a Maoist - Trotskyist block against an social democratic imperialism - stalinism (they're the same thing with extra steps) block.